Filters of this type have long been used to purify liquids such as water and wastewater of contaminants. The contaminants removed by these filters are undissolved and may be solids or immiscible liquids such as oil. For simplicity, the term ‘solids’ will be used herein and is intended to include immiscible liquid contaminants and solid contaminants. The simplest configuration consists of a single layer or bed of granular media such as silica sand, 30–100 cm in depth, contained within a containment means or vessel. The liquid to be filtered flows downward through the media bed under the influence of gravity or applied pressure.
As the service cycle proceeds, solids accumulate on the surface and within the media bed, in the interstices of the media particles. Solids are driven further into the media bed over time and eventually, they break through the media bed into the filtrate. At the same time, the flow resistance of the bed increases and the flow may become too slow and/or the pressure drop may become too high. The media must be cleaned or replaced when this occurs The simplest way to clean the media is by a process called ‘backwashing’.
During backwash, liquid, typically either feed or preferably filtrate, is passed up through the bed, expanding and fluidizing the filter media. Normally the overall depth of the media bed is expanded by 25–100% of its freely settled depth in this manner, thereby allowing the accumulated solids to be flushed from the filter. The effluent from the top of the vessel, containing the solid contaminants, is diverted to waste. At a minimum, the volume of backwash liquid must be greater than the void volume of the filter vessel. This void volume is equal to the volume of liquid above the media in addition to the interstitial volume of liquid in the filter media bed which is typically about 40% of the superficial media volume. Normally 2–5 void volumes of backwash water are required for an effective backwash to remove the majority of the solids. For a filter with 36 inches (91 cm) of media and 36 inches of freeboard, the void volume would be about 31.5 gallons per square foot (1.28 m3/m2) of filter area. Upon termination of the backwash flow, the media is allowed to settle by gravity before the filter is returned to service. The effectiveness of the backwash can be enhanced by simultaneously admitting air into the bed, providing provisions are made to avoid loss of filter media due to the increase in turbulence that this so-called ‘air scouring’ provides.
The liquid velocity for the backwash that is chosen, is dependent on the size and density of the media particles, but is typically about 15–25 gallons per minute per square foot of filter area (gpm/ft2) (37–61 m/h). A filter with 36 inches (91 cm) of media and a 36″ (91 cm) freeboard that is backwashed with 5 void volumes at 20 gpm/ft2 (48.9 m/h) would take approximately 8 minutes to backwash.
After backwash, the cleaned filter media particles re-settle by gravity to the bottom of the vessel. The largest diameter media particles, with the fastest terminal settling velocity, settle towards the bottom of the media bed, while the finest particles, with the slowest terminal settling velocity, settle towards to the top of the bed (assuming that all the particles have the same density). As a result, when the filter is placed in service once again, the feed liquid will see the finest media particles first, followed by progressively coarser media particles. This phenomenon is the opposite of what would be considered optimal, since most of the contaminant particles tend to filter on the top of the bed and not penetrate deeply into the bed.
The filter performance is dependent on a number of different parameters as far as its design and operation are concerned. The total suspended solids concentration [TSS] of the filtrate, for example is strongly influenced by the service liquid velocity and the particle diameter of the filtration media.
Higher service liquid velocities (i.e. service flow rates) tend to drive the solids further into the media bed. This results in utilization of a greater proportion of the media bed for solids retention, thereby increasing the solids loading capacity of the filter. On the other hand, a higher service liquid velocity generally causes increased leakage of solids into the filtrate. Liquid velocities of depth media filters are usually less than 8 gpm/ft2 (20 m/h) and typically about 4 gpm/ft2 (10 m/h).
By employing a finer media (i.e. smaller media particles) it is possible to reduce the [TSS] of the filtrate. The disadvantage of reducing the media particle size is that a larger proportion of the solids will be filtered on the top surface of the filter media bed instead of penetrating deeper into the media bed. As a result, pressure drop across the filter increases very quickly or the flow drops very rapidly. This results in prohibitively short filter runs between backwashes. The use of coarser media allows the solids to penetrate deeply into the filter media bed, thereby maximizing the solids loading capacity of the filter. The disadvantage of using coarse media is that the solids removal efficiency is inferior to fine media beds. The ideal filter design would have the coarsest filter media at the top of the filter bed with the media gradually decreasing in particle diameter with depth. In theory, this provides the high filtration efficiency of a fine media bed, along with the high solids loading capacity of a coarse media bed.
This objective can be approximated by using two or more media layers of differing specific gravity. For example, a so-called ‘dual media filter’, widely used in water treatment, contains approximately 30 inches (75 cm) of crushed anthracite (density=1.5, effective particle size=1 mm) over 6 inches (15 cm) of silica sand (density 2.7, effective size=0.35 mm). This concept has been extended in so-called ‘multi-media filters’ to employ as many as five different layers of media of decreasing effective size and increasing density.
Under conditions of optimal coagulation and system operation, dual and multi-media filters provide reasonably good performance. Even under optimal conditions however, such filters typically undergo a ripening period when they are first put into service after a backwash. During this period, water quality is inferior and must be discarded. Moreover, while the filtrate quality tends to initially improve as the cycle proceeds, termination of the cycle is usually necessitated by a [TSS] breakthrough (as evidenced by a turbidity increase). Sometimes, but not always, this [TSS] breakthrough is accompanied by an appreciably increased pressure drop and/or flow reduction.
Depth media filters are limited in their solids loading capacity. As the [TSS] of the feed water increases, the duration of the service cycle decreases. The proportion of the filtered water used for backwash and the quantity discarded due to filter ripening becomes an appreciable portion of the total production. Furthermore, the amount of time devoted to backwashing also becomes more significant as [TSS] increases. Eventually, feed [TSS] reaches a point where it is no longer feasible to use a filter of this type. Normally, feed [TSS] of about 50 ppm are considered the practical maximums. Although the solids loading capacity varies quite widely depending upon the nature and concentration of the feed [TSS] as well as the filter design and operating conditions, a good rule of thumb is about 1 pound of solids per square foot of filter media cross-sectional area or about 5 kg/m2.
As discussed above, single layer sand filters tend to accumulate a layer of solids on the top surface of the media bed. Once this layer is formed, most of the filtration occurs in this layer and not throughout the depth of the media bed. As a result, the service cycle may be relatively short. The solids accumulated in this top layer tend to become compressed together forming a crust over the duration of the filter service run. When the filter is backwashed, the crust breaks up, and forms large agglomerated fragments, that are much larger in size than the original particles being filtered. In some cases, these fragments are so large and dense that they cannot be removed from the filter during backwash. These fragments are sometimes referred to as ‘mud-balls’. Mudballs can accumulate in the filter media bed and eventually interfere with filtration operation and efficiency.
Various inventions have been devised to increase the solids loading capacity of depth media filters. U.S. Pat. Nos. 3,817,378 and 4,693,831 describe a single layer, gravity flow sand filter currently manufactured by US Filter under the trade mark HYDRO-CLEAR. In this filter, a pocket of air is introduced into the bottom underdrain system at sub-fluidization velocity to pulse the media bed between backwashings. According to these patents, “the pulsed fluid dislodges solids from the upper surface of the filter bed and folds a portion of them into the bed itself”. The manufacturer claims that this increases filter run length up to four times, greatly reducing the number of backwashes per day. An important feature of this filter is that solids are dislodged from the upper surface of the media bed, although no liquid is actually passed upwardly through the filter media bed during the so-called “back-pulse”.
The present invention aims to provide an improved method for of increasing the solids loading capacity of a depth media filter. This has the effect of increasing the amount of feed that can be treated between backwashes in a similar manner to the Hydro-Clear filter, however there are a number of significant and important differences which will become apparent.